270 



CHARACTER OF T II E 



About seven miles above the mouth of Nemakagon River, the St. Croix is ob- 

 structed for the distance of half a mile by ledges of red sandstone, which cross the 

 stream, forming rapids. The rock is in thin beds, and dips southeast at an angle of 

 12°. It is rather coarse-grained in some of the beds, and has thin angular frag- 

 ments of a very fine jasperoid rock embedded in it. It is principally made up of an- 

 gular grains of quartz, with but few rounded ones. In some places it has the 

 appearance of having been subjected to heat. The thicker beds are finer-grained 

 and more compact, and resemble the altered sandstones of Black River. 



Five miles above Pine Island, the river runs over beds of sandstone, dipping east 

 of south, at an angle varying from 5° to 7°. One mile further up, the dip is to the 

 southeast, and the rocks resemble, in all respects, the shaly sandstone beds of Lake 

 Superior. These rocks, which are very fine-grained and fissile, are also associated 

 with beds of red sandstone conglomerate. 



Above Pine Rapids, for the distance of fourteen or fifteen miles, the river is a 

 succession of rapids, only separated for short distances by broad, shallow basins of 

 comparatively still water, with numerous boulders scattered through them. Nearly 

 all the rapids above Nemakagon River are caused by accumulations of boulders, 

 which generally cross the stream in lines corresponding with the line of bearing, 

 and appear to have been arrested and formed into dams by the broken and uptilted 

 edges of beds of the red sandstone. From numerous instances which fell under my 

 observation, I am inclined to the opinion, that the disturbances accompanying the 

 eruption of the trap dikes, which traverse this region in a southwesterly direction, 

 produced numerous slight faults in the sedimentary rocks of that period, and thus 

 gave origin to the many sandstone dams which are now found crossing the rivers, 

 and producing rapids and basins in the way alluded to, and also to lakes of the 

 " first variety," as described in Chapter I. 



From a great number of observations, made not only on this but also on other 

 rivers, I am led to the conclusion that this is the general cause to which may be 

 attributed the formation of a majority of the rapids met with on the streams in this 

 section of country, and extending as far as the Mississippi, where the result of its 

 action is conclusively shown in the formation of the Falls of St. Anthony, which 

 bear every evidence of having been caused by a fault ; which fault was, in all pro- 

 bability, produced at the time of the eruption of the trap range which crosses the 

 St. Croix River at the Falls, and pursues a southwesterly course in the direction of 

 St. Anthony. 



Along the portion of St. Croix River under consideration, the banks are low, and 

 the bottom lands extend from one hundred and fifty to two hundred and fifty yards 

 on either side, to the base of the drift hills, which rise from one hundred to one 

 hundred and twenty feet in height, to the general level. The drift contains but 

 few large boulders, but many small ones, of granite, syenite, hornblende, and other 

 crystalline rocks, and numerous fragments of slate and red sandstone. Among 

 them, I found a small copper boulder. The country on the upland is barren, bear- 

 ing only clumps of small pine, a few bushes and stunted shrubs, coarse grass, and 

 wintergreen. 



A short distance above the point last designated, the shaly sandstone and its 



